1. Field of the Invention
The present invention is related to lifting mechanisms and, more particularly, is directed toward a lifting mechanism designed to hoist a dump bed from its chassis on a truck vehicle.
2. Description of the Prior Art
Many designs of lifting mechanisms for dump beds of trucks have been advanced, as exemplified by U.S. Pat. Nos. 3,572,521; 2,674,489; 3,317,062; and 2,640,725. The first-named Grey U.S. Pat. No. 3,572,521, believed closest in structure to the instant invention, illustrates a hydraulic cylinder 44 which is connected to a fixed bracket 50 on transfer table 38. The transfer table has an L-shaped extension pivoted at 40 to a bracket 42 which extends below load bed 16. The structure illustrated by Grey, however, requires and additional hydraulic cylinder 22 in order to pivot the load bed to its vertical position as shown in FIG. 1 thereof.
Most lifting mechanisms are designed in an attempt to achieve as great a force at the initiation of the lifting stroke as possible when the load bed must be moved from its horizontal rest postition. It is also generally desirable to provide a relatively constant leverage through the complete cycle of the stroke in order to minimize wear and tear on the working parts and to conserve power. Additionally, it is quite important to maintain leverage and power at the top of the stroke, near the end of the lifting cycle, in order to maximize the dump bed angle of inclination necessary with relatively heavy and/or wet loads, as can be appreciated by one skilled in the art.
Many of the prior art lifting mechanisms utilized for dump trucks fall short, in one or more of the foregoing aspects of achieving an ideal lifting cycle. In an attempt to provide a large initial force, many prior art lifting mechanisms employ one or more huge hydraulic cylinders, each of which may have two or three telescoping stages. Such hydraulic cylinders are bulky, quite expensive and are more prone to mechanical failure than less complex or smaller versions.
Further, and somewhat paradoxically, with the widely utilized two or three stage telescoping hoists, the lifting force is actually reduced as each of the second and third stages are employed. While many previous designs find it necessary to employ for example, a three stage telescoping cylindrical hoist to provide the requisite high final angle of inclination of the truck bed, as well as a high initial force, the subsequently reduced forces, especially near the top of the stroke, are highly disadvantageous, particularly when wet loads are being hauled which tend to remain stationary in the front of the dump bed and resist the normal tendency to slide rearwardly and be dumped.
Another problem encountered with prior art lifting mechanisms for dump beds is a tendency for the beds to tilt or twist about their longitudinal axis while being raised. This tilting or twisting tendency is particularly acute in those situations where heavy loads are being raised high by, for example, a pair of side-by-side three-stage hydraulic cylindrical hoists. If the load is not evenly distributed in the dump bed, or if the pair of outwardly telescoping cylinders do not extend evenly, twisting of the dump bed may result with concomitant damage.